Phenolic resins, which are thermosetting resins, are widely used as binders for bonding together materials that function as substrates, and because phenolic resins have excellent mechanical properties, electrical properties and adhesiveness, they are used in a wide variety of fields.
In order to obtain a molded item, a liquid phenolic resin is typically used by impregnating a substrate containing a fiber as a main component. Examples of this type of application include FRP, copper clad laminates, and wet paper friction materials.
Among such applications, the amount of friction materials using a phenolic resin as a binder is increasing in automobiles and railway carriages and the like. In particular, liquid resol-type phenolic resins are generally used for the friction materials used in the automatic transmission and the like of automatic vehicles, so-called wet friction materials. The properties demanded of these phenolic resins used in wet friction materials continue to increase year by year, and in particular, demands for increased flexibility of the phenolic resin have continued to grow, with the object of improving the coefficient of friction and the durability. However, despite having excellent mechanical properties, the cured products of typical phenolic resins tend to be hard and brittle, and cannot be said to exhibit excellent flexibility.
As a method of addressing the problems outlined above, tests are being conducted into improving the flexibility by using a drying oil or the like as a denaturant in the reaction used when synthesizing the phenolic resin (for example, see Patent Document 1).
However, this type of modified phenolic resin suffers a number of problems, including a marked deterioration in the strength after a heat history, and a short cycle life.